From: Benjamin Segall <bsegall@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, vincent.guittot@linaro.org,
linux-kernel@vger.kernel.org, juri.lelli@redhat.com,
dietmar.eggemann@arm.com, rostedt@goodmis.org, mgorman@suse.de,
bristot@redhat.com, corbet@lwn.net, qyousef@layalina.io,
chris.hyser@oracle.com, patrick.bellasi@matbug.net,
pjt@google.com, pavel@ucw.cz, qperret@google.com,
tim.c.chen@linux.intel.com, joshdon@google.com, timj@gnu.org,
kprateek.nayak@amd.com, yu.c.chen@intel.com,
youssefesmat@chromium.org, joel@joelfernandes.org, efault@gmx.de,
tglx@linutronix.de
Subject: Re: [PATCH 05/15] sched/fair: Implement an EEVDF like policy
Date: Fri, 29 Sep 2023 14:40:31 -0700 [thread overview]
Message-ID: <xm265y3sodyo.fsf@google.com> (raw)
In-Reply-To: <20230531124603.931005524@infradead.org> (Peter Zijlstra's message of "Wed, 31 May 2023 13:58:44 +0200")
Peter Zijlstra <peterz@infradead.org> writes:
> +
> +/*
> + * Earliest Eligible Virtual Deadline First
> + *
> + * In order to provide latency guarantees for different request sizes
> + * EEVDF selects the best runnable task from two criteria:
> + *
> + * 1) the task must be eligible (must be owed service)
> + *
> + * 2) from those tasks that meet 1), we select the one
> + * with the earliest virtual deadline.
> + *
> + * We can do this in O(log n) time due to an augmented RB-tree. The
> + * tree keeps the entries sorted on service, but also functions as a
> + * heap based on the deadline by keeping:
> + *
> + * se->min_deadline = min(se->deadline, se->{left,right}->min_deadline)
> + *
> + * Which allows an EDF like search on (sub)trees.
> + */
> +static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
> +{
> + struct rb_node *node = cfs_rq->tasks_timeline.rb_root.rb_node;
> + struct sched_entity *curr = cfs_rq->curr;
> + struct sched_entity *best = NULL;
> +
> + if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
> + curr = NULL;
> +
> + while (node) {
> + struct sched_entity *se = __node_2_se(node);
> +
> + /*
> + * If this entity is not eligible, try the left subtree.
> + */
> + if (!entity_eligible(cfs_rq, se)) {
> + node = node->rb_left;
> + continue;
> + }
> +
> + /*
> + * If this entity has an earlier deadline than the previous
> + * best, take this one. If it also has the earliest deadline
> + * of its subtree, we're done.
> + */
> + if (!best || deadline_gt(deadline, best, se)) {
> + best = se;
> + if (best->deadline == best->min_deadline)
> + break;
> + }
> +
> + /*
> + * If the earlest deadline in this subtree is in the fully
> + * eligible left half of our space, go there.
> + */
> + if (node->rb_left &&
> + __node_2_se(node->rb_left)->min_deadline == se->min_deadline) {
> + node = node->rb_left;
> + continue;
> + }
> +
> + node = node->rb_right;
> + }
I believe that this can fail to actually find the earliest eligible
deadline, because the earliest deadline (min_deadline) can be in the
right branch, but that se isn't eligible, and the actual target se is in
the left branch. A trivial 3-se example with the nodes represented by
(vruntime, deadline, min_deadline):
(5,9,7)
/ \
(4,8,8) (6,7,7)
AIUI, here the EEVDF pick should be (4,8,8), but pick_eevdf() will
instead pick (5,9,7), because it goes into the right branch and then
fails eligibility.
I'm not sure how much of a problem this is in practice, either in
frequency or severity, but it probably should be mentioned if it's
an intentional tradeoff.
Thinking out loud, I think that it would be sufficient to recheck via something like
for_each_sched_entity(best) {
check __node_2_se(best->rb_left)->min_deadline, store in actual_best
}
for the best min_deadline, and then go do a heap lookup in actual_best
to find the se matching that min_deadline.
I think this pass could then be combined with our initial descent for
better cache behavior by keeping track of the best rb_left->min_deadline
each time we take a right branch. We still have to look at up to ~2x the
nodes, but I don't think that's avoidable? I'll expand my quick hack I
used to test my simple case into a something of a stress tester and try
some implementations.
next prev parent reply other threads:[~2023-09-29 21:40 UTC|newest]
Thread overview: 104+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-31 11:58 [PATCH 00/15] sched: EEVDF and latency-nice and/or slice-attr Peter Zijlstra
2023-05-31 11:58 ` [PATCH 01/15] sched/fair: Add avg_vruntime Peter Zijlstra
2023-06-02 13:51 ` Vincent Guittot
2023-06-02 14:27 ` Peter Zijlstra
2023-06-05 7:18 ` Vincent Guittot
2023-08-10 7:10 ` [tip: sched/core] sched/fair: Add cfs_rq::avg_vruntime tip-bot2 for Peter Zijlstra
2023-10-11 4:15 ` [PATCH 01/15] sched/fair: Add avg_vruntime Abel Wu
2023-10-11 7:30 ` Peter Zijlstra
2023-10-11 8:30 ` Abel Wu
2023-10-11 9:45 ` Peter Zijlstra
2023-10-11 10:05 ` Peter Zijlstra
2023-10-11 13:08 ` Peter Zijlstra
2023-05-31 11:58 ` [PATCH 02/15] sched/fair: Remove START_DEBIT Peter Zijlstra
2023-08-10 7:10 ` [tip: sched/core] sched/fair: Remove sched_feat(START_DEBIT) tip-bot2 for Peter Zijlstra
2023-05-31 11:58 ` [PATCH 03/15] sched/fair: Add lag based placement Peter Zijlstra
2023-08-10 7:10 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2023-10-11 12:00 ` [PATCH 03/15] " Abel Wu
2023-10-11 13:24 ` Peter Zijlstra
2023-10-12 7:04 ` Abel Wu
2023-10-13 7:37 ` Peter Zijlstra
2023-10-13 8:14 ` Abel Wu
2023-10-12 19:15 ` Benjamin Segall
2023-10-12 22:34 ` Peter Zijlstra
2023-10-13 16:35 ` Peter Zijlstra
2023-10-14 8:08 ` Mike Galbraith
2023-10-13 14:34 ` Peter Zijlstra
2023-05-31 11:58 ` [PATCH 04/15] rbtree: Add rb_add_augmented_cached() helper Peter Zijlstra
2023-08-10 7:10 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2023-05-31 11:58 ` [PATCH 05/15] sched/fair: Implement an EEVDF like policy Peter Zijlstra
2023-08-10 7:10 ` [tip: sched/core] sched/fair: Implement an EEVDF-like scheduling policy tip-bot2 for Peter Zijlstra
2023-09-29 21:40 ` Benjamin Segall [this message]
2023-10-02 17:39 ` [PATCH 05/15] sched/fair: Implement an EEVDF like policy Peter Zijlstra
2023-10-11 4:14 ` Abel Wu
2023-10-11 7:33 ` Peter Zijlstra
2023-10-11 11:49 ` Abel Wu
2023-09-30 0:09 ` [PATCH] sched/fair: fix pick_eevdf to always find the correct se Benjamin Segall
2023-10-03 10:42 ` [tip: sched/urgent] sched/fair: Fix pick_eevdf() tip-bot2 for Benjamin Segall
[not found] ` <CGME20231004203940eucas1p2f73b017497d1f4239a6e236fdb6019e2@eucas1p2.samsung.com>
2023-10-04 20:39 ` [PATCH] sched/fair: fix pick_eevdf to always find the correct se Marek Szyprowski
2023-10-09 7:53 ` [tip: sched/urgent] sched/eevdf: Fix pick_eevdf() tip-bot2 for Benjamin Segall
2023-10-11 12:12 ` [PATCH] sched/fair: fix pick_eevdf to always find the correct se Abel Wu
2023-10-11 13:14 ` Peter Zijlstra
2023-10-12 10:04 ` Abel Wu
2023-10-11 21:01 ` Benjamin Segall
2023-10-12 10:25 ` Abel Wu
2023-10-12 17:51 ` Benjamin Segall
2023-10-13 3:46 ` Abel Wu
2023-10-13 16:51 ` Benjamin Segall
2023-05-31 11:58 ` [PATCH 06/15] sched: Commit to lag based placement Peter Zijlstra
2023-08-10 7:10 ` [tip: sched/core] sched/fair: " tip-bot2 for Peter Zijlstra
2023-05-31 11:58 ` [PATCH 07/15] sched/smp: Use lag to simplify cross-runqueue placement Peter Zijlstra
2023-08-10 7:10 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2023-09-12 15:32 ` [PATCH 07/15] " Sebastian Andrzej Siewior
2023-09-13 9:03 ` Peter Zijlstra
2023-10-04 1:17 ` [PATCH] sched/fair: Preserve PLACE_DEADLINE_INITIAL deadline Daniel Jordan
2023-10-04 13:09 ` [PATCH v2] " Daniel Jordan
2023-10-04 15:46 ` Chen Yu
2023-10-06 16:31 ` Daniel Jordan
2023-10-12 4:48 ` K Prateek Nayak
2023-10-05 5:56 ` [PATCH] " K Prateek Nayak
2023-10-06 16:35 ` Daniel Jordan
2023-10-06 16:48 ` [PATCH] sched/fair: Always update_curr() before placing at enqueue Daniel Jordan
2023-10-06 19:58 ` Peter Zijlstra
2023-10-18 0:43 ` Daniel Jordan
2023-10-16 5:39 ` K Prateek Nayak
2023-05-31 11:58 ` [PATCH 08/15] sched: Commit to EEVDF Peter Zijlstra
2023-06-16 21:23 ` Joel Fernandes
2023-06-22 12:01 ` Ingo Molnar
2023-06-22 13:11 ` Joel Fernandes
2023-08-10 7:10 ` [tip: sched/core] sched/fair: " tip-bot2 for Peter Zijlstra
2023-05-31 11:58 ` [PATCH 09/15] sched/debug: Rename min_granularity to base_slice Peter Zijlstra
2023-08-10 7:10 ` [tip: sched/core] sched/debug: Rename sysctl_sched_min_granularity to sysctl_sched_base_slice tip-bot2 for Peter Zijlstra
2023-05-31 11:58 ` [PATCH 10/15] sched/fair: Propagate enqueue flags into place_entity() Peter Zijlstra
2023-08-10 7:10 ` [tip: sched/core] " tip-bot2 for Peter Zijlstra
2023-05-31 11:58 ` [PATCH 11/15] sched/eevdf: Better handle mixed slice length Peter Zijlstra
2023-06-02 13:45 ` Vincent Guittot
2023-06-02 15:06 ` Peter Zijlstra
2023-06-10 6:34 ` Chen Yu
2023-06-10 11:22 ` Peter Zijlstra
2023-05-31 11:58 ` [RFC][PATCH 12/15] sched: Introduce latency-nice as a per-task attribute Peter Zijlstra
2023-05-31 11:58 ` [RFC][PATCH 13/15] sched/fair: Implement latency-nice Peter Zijlstra
2023-06-06 14:54 ` Vincent Guittot
2023-06-08 10:34 ` Peter Zijlstra
2023-06-08 12:44 ` Peter Zijlstra
2023-10-11 23:24 ` Benjamin Segall
2023-05-31 11:58 ` [RFC][PATCH 14/15] sched/fair: Add sched group latency support Peter Zijlstra
2023-05-31 11:58 ` [RFC][PATCH 15/15] sched/eevdf: Use sched_attr::sched_runtime to set request/slice Peter Zijlstra
2023-06-01 13:55 ` Vincent Guittot
2023-06-08 11:52 ` Peter Zijlstra
2023-08-24 0:52 ` [PATCH 00/15] sched: EEVDF and latency-nice and/or slice-attr Daniel Jordan
2023-09-06 13:13 ` Peter Zijlstra
2023-09-29 16:54 ` Youssef Esmat
2023-10-02 15:55 ` Youssef Esmat
2023-10-02 18:41 ` Peter Zijlstra
2023-10-05 12:05 ` Peter Zijlstra
2023-10-05 14:14 ` Peter Zijlstra
2023-10-05 14:42 ` Peter Zijlstra
2023-10-05 18:23 ` Youssef Esmat
2023-10-06 0:36 ` Youssef Esmat
2023-10-10 8:08 ` Peter Zijlstra
2023-10-07 22:04 ` Peter Zijlstra
2023-10-09 14:41 ` Peter Zijlstra
2023-10-10 0:51 ` Youssef Esmat
2023-10-10 8:01 ` Peter Zijlstra
2023-10-16 16:50 ` Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=xm265y3sodyo.fsf@google.com \
--to=bsegall@google.com \
--cc=bristot@redhat.com \
--cc=chris.hyser@oracle.com \
--cc=corbet@lwn.net \
--cc=dietmar.eggemann@arm.com \
--cc=efault@gmx.de \
--cc=joel@joelfernandes.org \
--cc=joshdon@google.com \
--cc=juri.lelli@redhat.com \
--cc=kprateek.nayak@amd.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mgorman@suse.de \
--cc=mingo@kernel.org \
--cc=patrick.bellasi@matbug.net \
--cc=pavel@ucw.cz \
--cc=peterz@infradead.org \
--cc=pjt@google.com \
--cc=qperret@google.com \
--cc=qyousef@layalina.io \
--cc=rostedt@goodmis.org \
--cc=tglx@linutronix.de \
--cc=tim.c.chen@linux.intel.com \
--cc=timj@gnu.org \
--cc=vincent.guittot@linaro.org \
--cc=youssefesmat@chromium.org \
--cc=yu.c.chen@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).